1. Field of the Invention
This invention relates to a robot having at least a plurality of movable legs and a method of controlling the motion of such a robot. More particularly, the present invention relates to a robot adapted to perform not only motions having only floor-touching periods such as walking but also those having both floor-touching periods and non-floor-touching periods such as running and jumping in an intermingled manner and also to a method of controlling the motion of such a robot.
Still more particularly, the present invention relates to a robot adapted to alleviate the load it is subjected to when one of the legs leaves or touches the floor in a running or jumping motion and also to a method of controlling the motion of such a robot. Still more particularly, the present invention relates to a robot adapted to alleviate the load it is subjected to when one of the legs leaves or touches the floor in a running or jumping motion by controlling the sole of the foot located at the front end of the leg and also to a method of controlling the motion of such a robot.
This application claims priority of Japanese Patent Application No. 2003-418988, filed on Dec. 17, 2003, the entirety of which is incorporated by reference herein.                                    2. Description of Related Art                        
A machine device adapted to move in a manner resembling to motions of human being by means of electric and/or magnetic operations is referred to as “robot”. It is the that the word “robot” derives from a Slavic word “ROBOTA (slave machine)”. In Japan, robots started to become popular in the later 1960s, although many of them were industrial robots such as manipulators and transfer robots installed for the purpose of automating productive operations of factories and saving man power.
Research and development efforts have been paid in recent years on mobile robots that are equipped with movable legs. Expectations are high for such mobile robots in practical applications. Two-legged mobile robots modeled after human motions are referred to humanoid robots.
While two-legged robots that are based on erect bipedalism are accompanied by problems of instability and difficulty of attitude control and walk control if compared with crawler type robots and four-legged or six-legged robots, they provide advantages including that they can adapt themselves to walking surfaces with obstacles and undulations on the way such as unleveled ground, and discontinued walking surfaces such as staircases and ladders to be stepped up and down so that they can move in a flexible manner.
Numerous techniques have already been proposed with regard to attitude control and stable walking of two-legged mobile robots. Stable “walking” as used herein is defined as “moving by using legs without stumbling and falling”. It is very important to perform attitude stabilizing for control preventing robots from stumbling and falling. When the body of a robot stumbles and falls, it means that the ongoing operation of the robot is suspended and that considerable power and time need to be spent for the robot to stand up from the falling state and resume its operation. Additionally, when the robot stumbles and falls, there arises a risk that the robot itself and/or the object that the falling robot collides with can be fatally damaged.
Many proposals relating to attitude stabilizing control and prevention of falling during a walk of legged mobile robots use a ZMP (zero moment point) as norm for judging the stability of walking of a two-legged mobile robot. The norm for judging the stability of walking using a ZMP is based on d'Alembert's principle that the gravity and the inertial force exerted by a walking system on the floor surface and their moments are balanced respectively by the reaction force of the floor exerted by the floor surface to the walking system and its moment. As a consequence of reasoning in the framework of dynamics, there exists a point where both the pitch axis moment and the roll axis moment are equal to zero, or a ZMP, on one of the sides of the supporting polygon (or the ZMP-stable region) formed by the contact points of the soles relative to the floor surface or in the inside thereof (see, inter alia, Non-Patent Reference Document 1).
In short, the ZMP norm provides that “a robot can stably walk without falling (as a result of a rotary motion of the machine body) if a ZMP is found inside the supporting polygon formed by the foot touching the floor and the floor surface and a force is exerted by the robot in a direction of pressing the floor surface”.
Generation of a bipedal walk pattern on the basis of the ZMP norm provides advantages including that it is possible to define a sole landing point in advance and that it is easy to consider kinetic restricting conditions for the tips of the feet corresponding to the profile of the floor surface. Additionally, using a ZMP for the norm for judging stability means that it is not force but a trajectory of motion that is handled as target for controlling the motion and hence it is technically more feasible.
Meanwhile, the legged mobile robots that have been developed so far are mostly designed to “walk” by making the robot constantly touch the floor surface at least at one of the legs thereof. In ordinary situations, walking is an optimal moving mode in which legged mobile robots can move safely and efficiently because it does not apply any excessive load to the mechanical and electric systems of the robot.
On the other hand, there may be situations that a legged mobile robot cannot cope with simply by means of its walking feature. In such a situation, the robot may be in a non-floor-touching state and hence cannot get any reaction force from the ground. When a legged mobile robot moves on the ground in a gravity-bound environment, it cannot be in a non-floor-touching state continuously for a long period of time, although there may be numerous situations where the robot is brought into a non-floor-touching state intermittently for a short period of time. For example, the robot may have to jump over a crevice of the ground, jump down from a step, run in order to move faster or change its stance by jumping up in order to maintain its bodily balance.
Although not many, there are some precursory researches that deal with balance control of legged mobile robots in a non-floor-touching state (see, inter alia, Non-Patent Documents 2 and 3). However, the outcome of those researches has not been actually utilized for the control systems of practical legged mobile robots.    (1) The degree of mechanical design freedom is limited to a large extent because the use of massless legs is assumed.    (2) Dynamics of transitions from walking to a halt have not been discussed sufficiently because only continuous jumping is assumed.    (3) It is difficult to define and impose geometric constraining conditions for the trajectories of the toes of the legs of the robot.
Techniques for computationally determining motion patterns of legged mobile robots in a situation where floor-touching states and non-floor-touching states coexist (see, inter alia, Non-Patent Document 4) have been proposed as extension of the ZMP norm. However, the technique proposed in the cited non-patent document entails a large computational load because it uses convergence operations to solve non-linear equations so that motion patterns have to be computed in advance on an off-line basis. In other words, it cannot generate motion patterns of coexistence of floor-touching periods and non-floor-touching periods for walking, running, jumping and other motions on a real time basis.
For walk control, the technique of computing motion patterns on the basis of the ZMP norm provides a basic control means that is highly promising for legged mobile robots today. The highest advantage of applying a ZMP to the norm for stability judgment of the machine body of a robot is its practicality including that it is easy to define and impose geometric constraining conditions for the trajectories of the toes of the legs of the robot and that it can be applied to broadly different machine models. For example, there are reports telling that techniques for generating walking patterns on a real time basis have been established so as make it possible for the robot to generate a stable motion pattern on board while walking or otherwise in motion (see, inter alia, Non-Patent Document 5).
On the other hand, for motions including non-floor-touching periods such as running and jumping motions, no practical control means has been proposed to date as pointed out above.
For instance, when a robot walks, jump or takes some other action, the load applied to each of the joints of the ankles and the other parts of the legs is enormous at the instant when the supporting leg leaves the floor or when the idle leg touches the floor. There is an idea of providing the legs of a robot with a buffering material or some other load absorption mechanism.
However, according to the concept of target ZMP follow-up control (as described above) that is in the mainstream of control method for bipedal robots, the soles of the robot are assumed to be formed by a rigid body and it is thought that the falling moment can be supported when a ZMP is found within the supporting polygon formed by the floor-touching points of the supporting leg and the floor. In other words, the use of a rigid body is desired for the soles. This contradicts the idea of providing the legs of a robot with a buffering material or some other load absorption mechanism.    [Non-Patent Document 1] Miomir Vukobratovic, “LEGGED LOCOMOTION ROBOTS” (Ichiro Kato et al., “Walking Robots and Artificial Legs”, (Nikkan Kogyo Shimbun, Ltd.)).    [Non-Patent Document 2] Kiyotoshi Matsuoka, “A Repetitive Jumping Model, Biomechanism 5” (University of Tokyo Press, 1985, pp. 4501–4509).    [Non-Patent Document 3] Marc H. Raibert et al., “Experiments in Balance with a 3D one-Legged Hopping Machine” (The International Journal of Robotics Research), 1984, Vol. 3, No. 2, pp. 75–92).    [Non-Patent Document 4] Kenichiro Nagasaka, “Generation of Whole Body Motions of a Humanoid Robot, Using Dynamics Filters” (1999, The Doctorial Dissertation, Information Engineering, Faculty of Engineering, The University of Tokyo).    [Patent Document 5] Letter of Specification, Japanese Patent Application Laid-Open Publication No. 2002-288745.